Method of producing large single crystals of mixed ferrites

ABSTRACT

The present invention relates to a method for producing a large single crystal of a mixed ferrite containing ZnO and having a spinel structure by melting the raw materials under an oxygen atmosphere with a pressure of from 1.5 to 20 atmospheres at a temperature of from 1,600* to 1,800*C to reduce the amount of ferrous ion contained in the molten ferrite to less than 7 wt. percent, and cooling the molten ferrite at the rate of from 2* to 8*C/hour to carry out the crystal growth by the Bridgman technique.

O United States Patent [191 [111 3,846,322, Sugimoto et al. Nov. 5, 1974 [54] METHOD OF PRODUCING LARGE SINGLE 3,027,327 3/1962 Blank 252/62.56

. 3,115,469 12/1963 Hamilton CRYSTALS F MIXED FERRITES 3,150,925 9/1964 Gambino 252/62.63 [76] Inventors: Mitsuo Sugimoto, No. 77, 3-chome Nakamachi, Nerima-ku, Tokyo; OTHER PUBLICATIONS Hiroshi Watanabe, 21 l, Tsuchidoi, Sendai-511i, MiYagi-ken, Sendai, Popova The Preparation of Single Crystals of Ferboth of Japan rites by the Vernevil Method, Soviets Physics Doklady 22 Filed: June 11 970 V01. 3, NO. 4, 1958, pages 711-712.

21 Appl. No.: 48,848

Primary Examiner-Oscar R. Vertiz Related Appllcatmn a Assistant ExaminerJ. Cooper [63] Continuation of Ser. No. 816,889, April 17, 1969,

abandoned, which is a continuation-in-part. of Ser. No. 456,666, May 18, 1965, abandoned. [57] ABSTRACT [30] Foreign Apphcatlon Pnomy Data The present invention relates to a method for produc- May 23, 1964 Japan 39-28833 ing a large Single crystal of a mixed ferrite containing May 23, 1964 Japan 39-28834 ZnO and having a Spine] Structure by melting the raw materials under an oxygen atmosphere with a pressure [52] US Cl. 252/62.62, 23/301 SP, 23/305 of from L to atmospheres at a temperature of [51] Int. Cl. C04b /30, C04b 35/38 from 1,600o to lsoooc to reduce the amount of Fleld of Search rous i contained i the molten ferrite to less than 7 23/301 305 wt. percent, and cooling the molten ferrite at the rate of from 2 to 8C/hour to carry out the crystal growth [56] References C'ted by the Bridgman technique.

UNITED STATES PATENTS 2,692,978 10/1954 Gait 252162.56 3 Claims, 12 Drawing Figures I600 soub LIQUID 6 A 2, I FezOs m 1400 [I I) p. q r: IELJ 1200 2 LL! IOOO Fes04 2O 4O FezOa MOL.%

PARTIAL OXYGEN PRESSURE (ATM) PAIENTEUIIIJV 5 I974 SHEET 10F 5 I600 so a A I LIQUID 9 I v Fe203 m I400 u: I) a: i I200 2 MAGNETITE (SS)+ HEMATITE P.

I I I I Fes04 20 4O 6O 8O FezOs CI LIQUID SOLID LIQUID F I 6.2 I

SOLID INVENTORS MlTsuo SUGIMOTO O l 1 I I HIROSHI WATANABE I660 I640 I620 I600 I580 I560 TEMPERATURE (C) ATTORNE PATENTEU W 5 1974 TEMPERATURE (C) $846322 sum an: 5

Ni Zn FERRITE Mn-Zn FERRITE I I l l Zn0 CONTENT (MOL.

FIG.6

1. METHOD OF PRODUCING LARGE SINGLE CRYSTALS OF MIXED FERRITES CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation of Ser. No. 816,889, filed Apr. 17, 1969, now abandoned which is a continuation-in-part application of the application Ser. No. 456,666 filed on May 18, 1965, now abandoned.

The present invention relates to a new and improved method of producing a large single crystal of a mixed ferrite with a spinel structure containing 2 30 mol percent zinc oxide and having the dimensions of at least about 20 mm in diameter and at least about 30 mm in length.

The development of a large single crystal of a mixed ferrite, such as MnZn, CuZn or NiZn ferrite, has been one of the most important problems. This is due to the reason that a core cut from a single crystal of a mixed ferrite containing zinc oxide has very good magnetic and electric characteristics. However, it is difficult to obtain a large single crystal of a mixed ferrite with a strictly stoichiometric composition containing zinc oxide. The main difficulties are that the evaporation of zinc and the reduction of ferric oxide (Fe O occur markedly at above the melting temperature of ferrites. The present invention is based on the discovery of new and improved techniques to overcome the aforementioned difficulties. The method of this invention comprises melting the raw materials of ferrite at a temperature of from 1,600 to 1,800C in an oxygen atmosphere with a pressure of from 1.5 to 20 atmospheres and subsequently cooling the molten ferrite at the rate of from 2 to 8 C/hour in an oxygen atmosphere with a pressure of from 1.5 to 20 atmospheres to carry out the crystal growth using a furnace of the Bridgman type. The mixture of oxides or compounds and the pulverized powder after sintering may be used for the raw materials; The detailed explanation of the present invention is as follows:

In a ferrite having a spinel structure, ferric oxide accounts for a large portion of its components, that is, about 70 wt. percent. When a ferrite is fired in air, ferric oxide is reduced to ferrous oxide (FeO) at a temperature higher than 1,300C, which is the dissociation temperature of ferric oxide, and the amount of ferrous oxide increases with increasing temperature. At a temperature higher than 1,600C, all kinds of ferrites having spinel structures are molten and contain a comparatively large amount of ferrous oxide. A small amount of ferrous oxide formed in the molten ferrite can be dissolved into the spinel structure during the cooling step and does not prevent the crystal growth. However, when a large amount of ferrous oxide is formed, even if it is dissolved in the molten ferrite, the secondary phase (Wiistite phase) will be precipitated during cooling. Accordingly, it is a very important problem for the crystal growth that when a mixed ferrite is molten, the formation of ferrous oxide is reduced as much as possible. The reduction of ferric oxide is remarkably affected by the oxygen pressure of the atmosphere.

In John M. Blank's article, Journal of Applied Physics; supplement 32, No. 3, March, 1961, 378S 379S and in US. Pat. No. 3,027,327 equilibrium atmosphere diagrams are reported which specify the schedules of temperature and percent oxygen by which ferrites can be cooled from the sintering temperature without net gain or loss of oxygen. These diagrams are clearly limited to. the temperature range of from 800 to 1,600C and they no longer apply if the ferrite melts or forms a second phase.

The difference of the equilibrium oxygen pressure depends on the factthat a ferrite is either solid or liquid, based on the binary system Fe O Fe O after Darken and Gury (Journal American Chemical Society, 68, 798 816 (1946)), as shown in FIG. 1.

That is, as shown in FIG. 2, the equilibrium oxygen pressure must follow a line a in the solid phase, a line b in the solid plus liquid phases and a line c in the liquid phase. Therefore, the equilibrium atmosphere after Blank in the solid phase (the line a in FIG. 2) can not be extrapolated to the liquid phase (a line a in FIG. 2).

As illustrated in FIG. 1, when the oxygen pressure of- .tures of mixed ferrites having spinel structures range from about 1,500 to about l,580C. Based on many investigations, it is found that the raw materials should be molten at a temperature of from 1,600 to 1,800C. under a elevated oxygen pressure of from 1.5 to 20 atms. to obtain a large single crystal of a mixed ferrite.

Thus, to study the reduction of ferric oxide in a mixed ferrite at a temperature higher than 1,600C, an induction furnace with the Bridgman structure as shown in FIG. 3 was designed, and it has been quite successful for the operation in oxygen atmosphere with a pressure up to 20 atms.

In the accompanying drawings,

FIG. 1 shows the binary system Fe O.,Fe O after Darken and Gury; 1

FIG. 2 shows the controlling method of the oxygen pressure of atmosphere according to the present invention;

FIG. 3 is a schematic representation of the structure of an induction furnace of the Bridgman type for growing single crystals according to the present invention;

FIG. 4 shows the amount of ferrous ion formed when Mn-Zn ferrite (mol ratio: 30 MnO/20 ZnO/SO Fe Og) is molten at 1,650C in an oxygen atmosphere at various pressures;

FIG. 5 shows the evaporation loss of zinc when MnZn ferrite is molten at 1,630C in an atmosphere of oxygen at various pressures;

FIG. 6 shows the experimental result on the optimum melting temperature for MnZn and NiZn ferrites when raw materials were molten in an oxygen atmosphere with a pressure of from 1.5 to 20 atms',

FIG. 7 shows a sample cut in the form of a plate along a preferred direction of magnetization from a single crystal;

FIG. 8 shows the rectangular hysteresis characteristics;

FIG. 9 shows the inside temperature gradient of the induction furnace in FIG. 1 (the Bridgman type);

FIG. 10 shows the oxidation rate of Fe Fe in Mn-Zn ferrite at 1,300C in air as measured from the increase by weight;

FIG. 11 shows the magnetic characteristics of a memory core cut from a single crystal of MnZn ferrite; and

FIG. 12 shows the electric properties of a memory core cut from a single crystal of MnZn ferrite.

In FIG. 3, l is a induction coil; 2 is a support for the induction coil; 3 is a suscepter made of Pt-Rh alloy (containing percent Rh) with a cylindrical hollow shape (40 mm in diameter, 420 mm in length and 2 mm in thickness) and with a thickness such that the highfrequency induction current (420 K.C.) is perfectly shielded and the molten ferrite is not agitated by the leakage current; 4 is a crucible made of Pt-Rh alloy (20 mm diameter X 50 mm). On growing the crystal, the induction coil is slowly pulled upward with a gear 5 while the crucible is rotated once per minute. 6 is a thermocouple and 7 is a autoclave vessel made of steel.

As shown in FIG. 1, the equilibrium oxygen pressure in the solid solution and in the liquid of Fe O has not been investigated. Then, the present inventors have studied the dissociation of Fe O at a higher temperature using the apparatus in FIG. 3.

It is seen from FIG. 4 that the amount of ferrous ion formed decreases as the oxygen pressure of the atmosphere increases. From an observation of microscopic structure, it was confirmed that the secondary phase (Wustite phase) appeared when the formation of ferrous ion became 7 wt. percent or more. Therefore, the crystal growth of a mixed ferrite should be carried out in an oxygen atmosphere of more than 1.5 atms. In the case of CuZn ferrite, the reduction of CuO-- Cu O can easily take place at a lower temperature, that is, the reduction progresses at about 1,050C or more in air. Then, an atmosphere of sufficiently high oxygen pressure is required for producing a single crystal of CuZn ferrite.

The second problem for producing a large single crystal of a mixed ferrite is the evaporation of zinc. It has been well known that the vapour pressure of zinc oxide is very large and zinc oxide in a mixed ferrite has the tendency of vaporizing as zinc at a higher temperature. Zinc content influences markedly the magnetic and electric properties of the produced single crystals. However, the evaporation of zinc in a mixed ferrite at a temperature higher than 1,600C has not heretofore been investigated.

FIG. 5 shows the amount of evaporation of zinc when Mn-Zn ferrite is kept at 1,630C for IS minutes. In the composition range with lower ZnFe O or at higher oxygen pressures, the percent evaporation of zinc shows a remarkable decrease. At a oxygen pressure of about more than 7 atms., it was very small.

The present inventors have investigated the minimum oxygen pressure necessary for producing a large single crystal of a mixed ferrite having good magnetic and electric properties, and found, as the result that a large crystal could well be produced by melting the raw materials of ferrite and subsequently cooling the molten ferrite, even at such a low oxygen pressure as 1.5

atms. There is a big improvement compared with a conventional melting process which is carried out in air (0.2 oxygen pressure). Further, it has been found that the reduction of ferric oxideand the evaporation of zinc decrease markedly when the oxygen pressure is about 20 atms. On the other hand, as the consumption of Pt or Rh of the crucible is increased at the higher oxygen pressures, it is desirable to carry out the crystal growth in the oxygen pressure range of about from 3 t 10 atms. from the commercial point of view.

With regard to the composition of a mixed ferrite of the present invention, zinc content is limited as follows:

From our investigation, it was found that when the amount of zinc contained in a mixed ferrite was less than 2 mol percent, the improvement of the magnetic properties was negligibly small. Furthermore, in case that the amount of zinc was more than 30 mol percent, the curie temperature of a core cut from a mixed ferrite fell down to below room temperature, so that the core could not be used in a practical application.

Therefore, from the above reason, the amount of zinc contained in a mixed ferrite of the present invention should be in the range of from 2 to 30 mol percent.

The third important problem for producing a large single crystal of a mixed ferrite is the melting temperature of raw materials of a ferrite. FIG. 6 shows the experimental result on the optimum melting temperature for MnZn and Ni-Zn ferrites when raw materials were molten in an oxygen atmosphere with a pressure of from 1.5 to 20 atms. In FIG. 6, the zero point on the horizontal axis indicates the composition of MnFe O or NiFe O (containing no ZnO).

In the case of MnZn ferrite, the optimum melting temperature was limited to the range of from l,600 to about I,730C and the melting temperature rose as the content of ZnO increases. And also, it was found that this temperature range can be applied to the case of CuZn ferrite. Furthermore, it is found that the melting temperature rises about 5C for the increase of one atmospheric oxygen pressure.

When raw materials of MnZn ferrite were molten at a temperature lower than l,600C, an aggregation of small crystals was produced. Therefore, raw materials of MnZn ferrite should be molten at a temperature higher than 1,600C to obtain a large single crystal of a mixed ferrite.

In the case of Ni-Zn ferrite, the optimum melting temperature is from about 1,700 to about l,800C and the temperature falls as the content of ZnO increases. As the consumption of Pt or Rh of the crucible and the evaporation of zinc are remarkable at a temperature higher than 1,800C, it is desirable to melt raw materials of a ferrite at a temperature lower than 1,800C.

The fourth important problem for producing a large single crystal of a mixed ferrite is the rate of cooling the molten ferrite for growing the crystals. When the molten ferrite was cooled very quickly, that is, at a rate of more than 8C/hour, many cracks were formed in the produced crystal by the thermal shock of such quick cooling, and an aggregation of small crystals was produced. Hence, the rapid cooling of the molten ferrite is not satisfactory for the growth of a large single crystal. Furthermore, it was discovered that at a cooling rate of less than 2C/hour, the influence of the cooling speed on the crystal growth was entirely negligible, and such slow cooling speed was not convenient for the production of a large single crystal from the industrial point of view. Then, it is concluded that for the production of a large single crystal of a mixed ferrite, having the dimension of at least about mm in diameter and at least about mm in length, the raw materials should be molten under an oxygen atmosphere with a pressure of from l.5 to 20 atms., and the molten ferrite should be cooled at the rate of from 2 to 8C/hour in an oxygen atmosphere with a pressure of from 1.5 to 20 atms.

The just grown single crystal should be cooled very slowly to room temperature in equilibrium with the oxygen pressure of the atmosphere so that it has no net gain or loss of oxygen and no modification of the crystal structure.

In the method of this invention, the influence of the oxygen pressure on the grown single crystal during cooling to room temperature was negligibly small, because many parts of the crystal are closely covered with the crucible made of PtRh alloy, except for the upper.

surface. Furthermore, as the produced crystal was very dense and the diffusion rate of oxygen into the said crystal was very slow, the oxidation of the said crystal, except for the upper surface was negligible.

The mixed ferrite produced by the method of the present invention includes, of course, mixed ferrites having a little deviation from the strictly stoichiometric composition.

A single crystal of a mixed ferrite produced by the method of the present invention has the advantage that the magnetic and electric properties can be provided as desired by proper selection of the kind and composition of ferrite. A single crystal of a mixed ferrite containing zinc oxide of the present invention has properties superior to those of a single crystal of a conventional single ferrite or polycrystal ferrite. Therefore, the mixed ferrite is quite useful as a core material in electronic application. Moreover, since its hysteresis loop is rectangular, the core of a single crystal cut from a large crystal can be used as a memory core of a computer. Many memory cores with the same properties can be produced from a parent large single crystal at the same time. It was found that when a single crystal of a mixed ferrite containing ferrous ion in an amount of less than 7 wt. percent of the total amount of iron, was reheated at temperature below 1,300C and reoxidized in an oxidizing atmosphere, the amount of ferrous ion could be controlled without causing any modification of the crystal structure. As it is calculated that the number of oxygen vacancies in such a reducing state corresponds to less than one hole per unit cell of the spine] structure, so that no modification of crystal appeared during oxidizing process. It is found that Laue spots of the said crystal have become very small and sharp after re-heating. This shows that regularity of the crystal has been increased due to the removel of the oxygen vacancies.

Next, the process of cutting a single crystal of a mixed ferrite along a preferred direction of magnetization to obtain a ferrite core having rectangular hystere sis characteristics is explained in detail.

In general, the preferred direction of magnetization of spinel-type ferrites containing no Co is in the direction of (111) which it is in the direction of 100) in the case of Co ferrite or spinel-type ferrites containing Co as one component. The plate should be cut off along the preferred direction of magnetization (111) for a ferrite containing no Co and along the direction of for a ferrite containing Co as shown in FIG. 7. Then, a ferrite core with a ring shape'or a diamond shape is cut from the plate shape specimen by an appropriate method (for example, the supersonic technique, the etching method, etc.)

As mentioned above, a memory core of a single crystal of a mixed ferrite obtained by the method of the present invention containing the preferred direction of magnetization has the following excellent characteristics, compared with a ferrite core of a polycrystal specimen prepared by a conventional sintering method.

1. A single crystal core of the mixed ferrite prepared by the method of the present invention has an ideal rectangular hysteresis characteristic, compared with a ferrite core of a polycrystal prepared by a conventional sintering method. The rectangular hysteresis characteristics are represented, as shown in FIG. 8, in terms of the ratio of the residual flux density (Br) to the saturation flux density (Bm), i.e., Br/Bm. From the practical point of view, the ratio of the flux density B at the magnetic field Hm/2 which is slightly smaller than the coercive 'force (I-lc) to the maximum flux density Bm at the magnetic field Hm, i.e., B /l3m, is often used; The nearer the ratios approach unity, the more the hysteresis curve becomes rectangular. In case of a ferrite core of a conventional polycrystal, the abovementioned ratio is about 0.93 0.95 and it is difficult to produce a ferrite core having a ratio more than 0.95. However, in the case of a single crystal core of a mixed ferrite of the present invention, a ratio of about 0.98 is easily attained.

2. In the case of a single crystal core of a mixed ferrite prepared by the method of the present invention, the switching time is short and the driving power is small. On the contrary, in case of a ferrite core of a polycrystal prepared by a conventional sintering method, the kind and the composition of the ferrite should be strictly limited to a composition of MnMg ferrite. Therefore, it is difficult to change the switching time and the driving current in a wide range. In the case of a single crystal core of a mixed ferrite of the present invention, since it is cut from a single crystal of a large mixed ferrite along the preferred direction of magnetization, the rectangular hysteresis characteristic becomes more ideal. Moreover, various switching times and driving powers are attained by changing the kind and the composition of the ferrite employed. For example, when a large amount of Zn ferrite is dissolved, it is possible to make the He and the driving power smaller. As one of the electric properties of a memory core cut from a single crystal of MnZn ferrite, a switching time of less than 0.4 11. second was attained as shown in FIG. 12. In case of a core of the conventional polycrystal ferrite, when a large amount of Zn ferrite is dissolved, the rectangular hysteresis characteristic tends to be lost appreciably. On the other hand, in the case of a single crystal core of a mixed ferrite of the present invention, even if a large amount of Zn ferrite is dissolved, the rectangular hysteresis characteristic is not reduced at all, and a ferrite core having an excellent rectangular hysteresis characteristic and quite small driving power, which can not be attained by a conventional method, can be produced. A ferrite core which has been cut from a single crystal of a conventionally known single ferrite, such as Cu, Mn, Ni or Zn ferrite in the preferred direction of magnetization is not so'desirable. That is to say, a ferrite core prepared from a single crystal of a conventional single ferrite has the rectangular hysteresis characteristic, but it is difficult to make the driving power and the switching time as small as in the case of a core of a single crystal of a mixed ferrite prepared by the method of the present invention.

3. A plurality of single crystal cores of the mixed ferrite prepared by the method of the present invention have such characteristics that each specimen is uniform in its size and the magnetic and the electric properties since each is cut mechanically from the same large single crystal. It has been a very difficult problem from the commercial point of view in the case of a core of polycrystal prepared by a conventionally known sintering method. The single crystal of the present invention has such characteristics that for example, about 50,000

' pieces of ferrite cores having the same properties and the size of 50 mils (outer diameter, about 1.27 mm; thickness, about 0.38 mm) can be obtained from a single crystal of the ferrite having a diameter of about 30 mm and a length of 50 mm.

The present invention is explained in detail with the following example. In these example, the composition of the ferrites is given in terms of mol ratio of oxides.

EXAMPLE 1 MnZn ferrite (Composition: 30 MnO/ ZnO/50 Fe O was made molten at l,650C in an oxygen pressure of 3 atms. by means of the apparatus shown in FIG. 3. Then, the molten ferrite was cooled down to 1,500C at the rate of 2C/hour following the temperature gradient as shown in FIG. 9 in the same atmosphere. As the result, a large single crystal of a mixed ferrite of 20 X 30 mm was obtained, i.e., 20 mm diameter X 30 mm length.

EXAMPLE 2 About 35 grams of ferrite (composition: NiO/25 ZnO/50 Fe O was put in platinum-rhodium crucible 4 in FIG. 3, heated at 1,680C (in an oxygen pressure of 3.5 atms.) and made molten. Thereafter, it was cooled slowly to 1,5 50C at the rate of 8C/hour. As the result, a large single crystal of 20 X 35 mm was obtained, i.e., 20 mm diameter X 35 mm length.

EXAMPLE 3 A single crystal of MnZn ferrite of the invention containing about 7 wt. percentof Fe was divided into 4 pieces as shown in FIG. 10, and heated at 1,300C in an atmosphere of air for about 20 hours. FIG. 10 shows the oxidation rate of Fe in Mn-Zn ferrite at 1,300C in air, as measured from increase of the weight. As is clear from the Figure, most of the Fe could be reoxidized by such heating for about 20 hours even at such a low temperature. Before reheating, the Laue photograph of the specimen containing Fe showed a spinel phase but the spots were big and obscure. However, after reheating, the spots were very sharp.

EXAMPLE 4 Electrolytic iron, electrolytic manganese and electrolytic zinc were weighted to form a composition of MnO/20 Zno/50 Fe O and then dissolved in nitric acid.

After drying, a powder mixture of oxides was obtained. The resulting powder was put in the Pt-Rh crucible 4 in FIG. 3, and it was made molten at l,640C in an oxygen pressure of 5 atms. The molten ferrite was cooled at the rate of 5C/hour in an oxygen pressure of 3 atms. A single crystal having the dimension of about 20 mm in diameter and about 30 mm in length was obtained.

It was found, as the result of chemical analysis, that the single crystals contained about 5 wt. percent of Fe. The preferred direction of magnetization (111) was determined by the X-ray Laue method, and plateshaped specimens having a thickness of 0.6 mm were cut by a precision diamond cutter including the direction of (111). Ring and diamond-shaped cores of single crystals having about 5 mm outer diameter were cut from the plate-shaped specimens by employing a supersonic cutting machine. According to this experiment, 5 pieces of ferrite core of single crystals were obtained from the crystal at the same time. These cores had characteristics very similar to one another.

FIG. 11 shows the result of direct observation of the hysteresis curve of such a core using a Brown tube. It was found that the core had excellent rectangular hysteresis characteristics.

The magnetic properties were as follows:

Residual magnetism Br 3,100 gauss Coercive force He 0.75 oersted Rectangularity ratio Br/Bm 0.98

Rise up time of pulse current 0.2 ;1. sec. Pulse width 20 y. sec. Time pitch of pulse 55 p. sec. Disturb ratio 0.5

Measuring temperature From the Figure, it is easily found that the driving power is small and the switching time is short.

We claim:

1. A method for producing a large single crystal of a mixed ferrite which is a member selected from the group consisting of manganese-zinc ferrite and nickel zinc ferrite with a spinel structure, containing zinc oxide in the amount of between 2 and 30 mole per cent, which comprises melting raw materials for the ferrite in a heating zone at a temperature above l,600 to 1,800C in an atmosphere consisting essentially of oxygen having a partial pressure of oxygen of from 1.5 to 20 atmospheres to reduce the content of ferrous ion in the molten ferrite to less than 7 wt. percent of the total amount of iron and subsequently cooling the molten ferrite at the rate of 2 8C/hour by slowly moving said molten ferrite relative to and out of said heating zone in an atmosphere consisting essentially of oxygen having a partial pressure of oxygen of from L5 to 20 atmospheres to carry out the crystal growth, whereby a single crystal having the dimension of at least about 20 mm in diameter and at least about 30 mm in length is produced.

2. A method according to claim 1, wherein the single crystal with the spinel structure and containing ferrous ion in amount of less than 7 wt. percent of the total amount of iron thus produced is reheated at a tempera- 9 10 ture below 1,300C in an oxidizing atmosphere to regen atmosphere employed during melting and during duce and control the amount of ferrous ion without the process of the crystal growth is at-a pressure of from causing any modification of the crystal structure. 3 to 10 atmospheres.

3. A method according to claim 1, wherein the oxy- UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3846322 D t d November 5, 1974 Inventoflg) Mitsuo Sugimoto and Hiroshi Watanabe It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

As signee: RIKAGAKU KEN'KYUSHO Tokyo, Japan Signed and Scaled this eighteenth D 21) 0f November I 9 75 [SEAL] A ttes t:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (umml'ssjnmr 0] Patents and Tratlwnurkx 

1. A METHOD FOR PRODUCING A LARGE SINGLE CRYSTAL OF A MIXED FERRITE WHICH IS A MEMBER SELECTED FROM THE GROUP CONSISTING OF MANGANESE-ZINC FERRITE AND NICKEL-ZINC FERRITE WITH A SPINEL OF STRUCTURE, CONTAINING ZINC OXIDE IN THE AMOUNT OF BETWEEN 2 AND 30 MOLE PER CENT, WHICH COMPRISES MELTING RAW MATERIALS FOR THE FERRITE IN A HEATING ZONE AT A TEMPERATURE ABOVE 1,600* TO 1,800*C IN AN ATOMPHERE CONSISTING ESSENTIALLY OF OXYGEN HAVING A PARTIAL PRESSURE OF OXYGEN OF FROM 1.5 TO 20 ATOMOSPHERES TO REDUCE THE CONTENT OF FERROUS ION IN THE MOLTEN FERRITE TO LESS THAN 7WT. PER CENT OF THE TOTAL AMOUNT OF IRON AND SUBSEQUENTLY COOLING THE MOLTEN FERRITE AT THE RATE OF 2* -8*/HOUR BY SLOWLY MOVING SAID MOLTEN FERRITE RELATIVE TO AND OUT OF SAID HEATING ZONE IN AN ATOMPHERE CONSISTING ESSENTIALLY OF OXYGEN HAVING A PARTIAL PRESSURE OF OXYGEN OF FROM 1.5 TO 20 ATMOSPHERES TO CARRRY OUT THE CRYSTAL GROWTH, WHEREBY A SINGLE CRYSTAL HAVING THE DIMENSION OF AT LEAST ABOUT 20 MM IN DIAMETER AND AT LEAST ABOUT 30 MM IN LENGTH IS PRODUCED.
 2. A method according to claim 1, wherein the single crystal with the spinel structure and containing ferrous ion in amount of less than 7 wt. percent of the total amount of iron thus produced is reheated at a temperature below 1,300*C in an oxidizing atmosphere to reduce and control the amount of ferrous ion without causing any modification of the crystal structure.
 3. A method according to claim 1, wherein the oxygen atmosphere employed during melting and during the process of the crystal growth is at a pressure of from 3 to 10 atmospheres. 